Digital systems, such as digital switching and transmission systems, require an accurate source of clock pulses to synchronize and control their operation. In a stand alone environment in which a digital system is not connected to other systems, the clock pulses may be provided by a source internal to the system. However, as is more typically the case, a digital system is often connectable to and/or must communicate with other digital systems each of which also has its own internal clock source. It is necessary when two or more digital systems communicate with each other that one system send timing information to the other system to synchronize the timing of the two systems. This synchronization is necessary to prevent the loss or mutilation of data transferred between the two systems.
It is therefore necessary that digital systems be capable of being controlled by different timing sources at different times. This requires that the system be controlled by its own internal clock source at certain times and, at other times, that it be controlled from a clock source of one of other systems with which it can communicate.
The switching of a digital system from one clock source to another presents problems with regard to the frequency and phase relationship of the various clock sources. The frequency and phase of these various sources must be precisely controlled so that when a switch is made between sources, there is a minimum of transients in the resulting clock signal received by a controlled system. This is necessary if there is to be a minimal loss or corruption of the data signals then being served by the controlled system.
An obvious solution to this problem would be to equip the various potential clock sources with precision clock circuits of exactly the same frequency and phase so that the controlled system will be oblivious to a timing change when it is switched from one source to another. It is not economically feasible to provide a plurality of clock sources having such frequency and phase capabilities. The achievement of the required frequency stability between a plurality of sources is perhaps attainable. However, it is a problem to keep a plurality of sources in phase synchronization. This is particularly the case in situations where a first one of the sources may comprise a part of the system to be controlled while one or more of the other sources is external to the controlled system and connectable thereto over comunication lines of varying network patterns and lengths.
The length of the communication line interconnecting the remote source with the controlled system is a determining factor regarding the phase of the remote source signal as seen by the controlled system. It is therefore difficult to ensure that no phase differences are encountered when switching from the local clock source to the remote source or vice versa. This difficulty is compounded by the fact that the remote source and the local system may be connectable by different lines at different times with the different lines having different phase and transmission characteristics. Also, it is not feasible to control the phase of the remote source at the controlled system since the same source is independently operated and may be concurrently providing clock signals for a plurality of remotely situated digital systems.
In summary, it is a problem to provide a plurality of clock sources having the identical frequency and phase characteristics so that when the control of a digital system is switched from one source to another, that no frequency or phase disturbances are encountered by the controlled system.